Write the function calculateSD that calculates the standard deviation of 4 numbers (double). Basic steps to calculate standard deviation are: a. Work out the Mean (the simple average of the numbers) b. Then for each number: subtract the Mean and square the result - this is called variance. c. Then work out the mean of those squared differences. d. Take the square root of that and we are done! Hint: Define a simple Average function as you can use it for steps a and c.